A. Field of Invention
This invention relates generally to a rate-responsive pacemaker, and, in particular, to a rate-responsive pacemaker in which the rate control parameter is minute volume and the sensing is accomplished by dual chamber unipolar leads.
B. Description of the Prior Art
U.S. Pat. No. 4,702,253, entitled "Metabolic-Demand Pacemaker and Method of Using the Same to Determine Minute Volume", which issued Oct. 27, 1987, to Nappholz et al., discloses a rate-responsive pacemaker which employs minute volume as a rate control parameter. Minute volume is a measure of the amount of air inspired by a person as a function of time. The greater the amount of air inspired, the greater the need for a higher heart pacing rate. The pacemaker of the aforesaid patent (hereinafter called "the '253 pacemaker") measured minute volume by providing a three-electrode lead which employs one electrode referenced to a pacemaker case to sense heart signals and pace the patient's heart in the conventional manner and which employs the remaining two electrodes to perform the minute volume measurement. The two electrodes for measuring minute volume were located in the superior vena cava blood vessel and/or in a cardiac chamber in the vicinity of the patient's pleural cavity. The '253 pacemaker periodically applied current pulses between one of the electrodes and the pacemaker case, and measured the voltage which resulted from the applied current between the other electrode and the pacemaker case. The measured voltage was a function of the blood impedance in the vessels in and around the pleural cavity which, in turn, depended upon the pleural pressure. The '253 pacemaker determined the minute volume by monitoring the variation in the impedance measurement.
One problem with the '253 pacemaker is that it requires a lead having at least three electrodes while the industry has standardized unipolar (single electrode) and bipolar (dual electrode) leads. There are many patients with implanted old pacemaker systems having unipolar and bipolar leads, and if a three-electrode lead is required for a new pacemaker, then the new pacemaker cannot readily replace an old pacemaker and use the old leads. Furthermore, there are physicians who like the feel of the existing, standard leads they have been using in the past, and one factor which weighs against implanting a rate-responsive pacemaker might be that it requires a new lead type having a new feel.
U.S. Pat. No. 4,901,725, entitled "Minute Volume Rate-Responsive Pacemaker", which issued Feb. 20, 1990, to Nappholz et al., disclosed an improved minute rate-responsive pacemaker (hereinafter called "the '725 pacemaker") which can be used with a conventional bipolar lead. This bipolar lead had two electrodes for sensing and pacing the heart. In the '725 pacemaker, the standard ring electrode was used additionally to apply a current which flows to the pacemaker case. The tip electrode was used to measure the blood impedance between the tip and the case in response to the current pulse applied through the ring electrode. The '725 pacemaker utilized the measured blood impedance to derive an appropriate pacing rate.
Although the '725 pacemaker used a bipolar lead which is standard in cardiac pacing, it had a limitation in that this pacemaker could not be used in the many patients who had previously implanted unipolar leads. Unipolar leads, which have a single tip electrode, are also standard in the art of cardiac pacing. If a bipolar lead is required when a patient has a new pacemaker implanted, then a non-rate-responsive pacemaker that is connected to a unipolar lead cannot be replaced by a rate-responsive pacemaker embodying the invention of the '725 pacemaker simply by exchanging pacemakers and using the same lead.
Previous attempts have been made to perform minute volume rate-responsive pacing in a pacemaker using unipolar leads. These attempts failed, primarily because the blood impedance signal measured from unipolar leads was too weak in comparison to system noise and other unwanted signals present. Additionally, U.S. Pat. No. 5,201,808 (hereinafter called "the '808 pacemaker") entitled "Minute Volume Rate-Responsive Pacemaker Employing Impedance Sensing on a Unipolar Lead," which issued Apr. 13, 1993, to Steinhaus et al., discloses calculating minute volume through the use of one unipolar lead. The '808 pacemaker did not measure impedance between the tip electrode and the case as was previously done in prior art devices, but instead the impedance measurement was taken between the input of the lead from the pacemaker and the pacemaker case. Therefore the pacemaker senses the impedance of the body tissues along the entire length of the lead by providing a high frequency exultation to the lead. Therefore the pacemaker requires very complex circuitry.
Accordingly, it is desirable to develop a minute volume rate-responsive pacemaker device for use with conventional unipolar leads that does not significantly increase the complexity of the circuitry involved, and does not increase the cost of manufacture of the device.